Gas Dehydration
Chapter 11
Based on presentation by Prof. Art Kidnay
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
Plant Block Schematic
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
2
Topics
• Introduction
• Water Content of Hydrocarbons
• Gas Dehydration Processes
 Absorption processes
 Adsorption processes
 Non regenerable desiccant processes
 Membrane processes
 Other processes
 Comparison of dehydration processes
• Safety and Environmental Considerations
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
3
Topics
• Introduction
• Water Content of Hydrocarbons
• Gas Dehydration Processes
 Absorption processes
 Adsorption processes
 Non regenerable desiccant processes
 Membrane processes
 Other processes
 Comparison of dehydration processes
• Safety and Environmental Considerations
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
4
Reasons for Gas Dehydration
• Field Operations
 Prevent hydrate formation
 Minimize corrosion
Need to dry gas to dew point below lowest operating temperature
• Plant Operations
 Need to have less than 1 ppmv H2O in gas to cryogenc units
 Need 4 to 7 lb/MMscf (85 to 150 ppmv) in pipeline
Glycol dehydration most common to produce water contents > 10 ppmv
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Equilibrium considerations
• Equal fugacities for each component in each phase. Between gas & water
phases:
 P  vi ,L  
φi ,L  γ i Pi vap 
yi xi K i where =
Ki = 
dP 
=
 exp  ∫ 

φi ,V  φi ,V P 
 Pisat  RT  
• For a gas in contact with pure water:
vap
PH2O
yH2O ≈
P
• The formation of a water phase will control the mole fraction of water in the gas
phase.
• Increasing water in the feed increases the amount of free water, not the
concentration of water in the gas.
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Water content of natural gas
• Based on “typical” gas composition
 Separate corrections for actual
composition (gas specific gravity) & acid
gas content
• Takes into account non-idealities in the
gas phase
• Take care if gas is specified as “wet” or
“dry” basis – dry basis does not include
the amount of water in the MMscf
NH2O / MH2O
=
yH2O / MH2O
NHC + NH2O
N / MH2O yH2O / MH2O
=
Dry Basis: XH2O ⇒ H2O
NHC
1 − yH2O
Wet Basis: XH2O ⇒
 At very low water contents these values
are essentially the same
Fig. 20-4, GPSA Engineering Data Book, 13th ed.
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7
Water content of natural gas – typical pipeline specs
GPSA Engineering Data Book, 13th ed.
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8
Water content of natural gas
GPSA Engineering Data Book, 13th ed.
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9
Applicability of dehydration processes
1000
Wet Gas Water Content, lb/MMscf
Compression and
Cooling
100
Liquid Desiccants
Glycol & Methanol
Enhanced Glycol
Alumina
and
Silica Gel
10
Molecular
Sieves
1
-80
-60
-40
-20
0
20
40
60
80
100
120
140
Dry Gas Water Dew-Point, degrees F
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
10
Topics
• Introduction
• Water Content of Hydrocarbons
• Gas Dehydration Processes
 Absorption processes
 Adsorption processes
 Non regenerable desiccant processes
 Membrane processes
 Other processes
 Comparison of dehydration processes
• Safety and Environmental Considerations
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
11
Physical absorption
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Equilibrium considerations
• Glycols tend to be only in the water phase (i.e., non-volatile, very low solubility
in the hydrocarbon liquid phase
• For a gas in contact with water/glycol mixture:
vap
PH2O
′
yH2O ≈ xH2O
P
• Water content in the gas phase less than that for a pure water phase since
x’H2O < 1
• Away from glycol, must reduce temperature to create a free water phase.
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13
Glycols typically used
EG
DEG
TEG
Ethylene Glycol
Diethylene Glycol
Triethylene Glycol
C2H6O2
C4H10O3
C6H14O4
Molecular Weight
62.1
106.1
150.2
Boiling Point (°F)
386.8
473.5
550.4
Vapor pressure @ 77°F (mmHg)
0.12
< 0.01
< 0.01
Density @ 77°F (lb/gal)
9.26
9.29
9.34
Viscosity @ 77°F (cP)
16.9
25.3
39.4
Decomposition temperature (°F)
329
404
328
Name
Formula
Fig. 20-50, GPSA Engineering Data Book, 13th ed.
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14
Glycol molecular structure
• Ethylene glycol
HO-CH2-CH2-OH
• Diethylene Glycol
HO-CH2-CH2-O-CH2-CH2-OH
• Triethylene Glycol
HO-CH2-CH2-O-CH2-CH2-O-CH2-CH2-OH
Chemical structures drawn using
http://molview.org/
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Equilibrium water content for TEG solutions
Fig. 20-59, GPSA Engineering Data Book, 13th ed.
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Equilibrium water content for TEG solutions
Fig. 20-59, GPSA Engineering Data Book, 13th ed.
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Example Glycol Dehydration Unit
Fig. 20-58, GPSA Engineering Data Book, 13th ed.
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Typical Glycol Dehydration Unit
• System
 2 – 5 gal TEG per lb water removed
• Absorber / Contactor
 60 – 100oF inlet
 Can operate up to 2,000 psia
 Typically 4 – 10 bubble cap trays
• 25 – 30% efficiency
 5 – 10 psi pressure drop
• Flash tank
 10 – 20 minute residence time
 150oF, 50 – 75 psig
• Regenerator
 Packed equivalent to 3 – 4 trays
 375 – 400oF
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
19
Typical Glycol Dehydration Unit
• System
 2 – 5 gal TEG per lb water removed
• Absorber / Contactor
 60 –
100oF
Regenerator Temperatures
inlet
 Can operate up to 2,000 psia
 Typically 4 – 10 bubble cap trays
• 25 – 30% efficiency
 5 – 10 psi pressure drop
• Flash tank
 10 – 20 minute residence time
 150oF, 50 – 75 psig
• Regenerator
Absorber Temperatures
 Packed equivalent to 3 – 4 trays
 375 – 400oF
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
vap
yH2OP ≈ xH2OPH2O
20
Solubility of hydrocarbons in glycol solutions
GPSA Engineering Data Book, 13th ed.
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
21
Field Glycol Dehydrator
contactor
stripper
glycol pump
reboiler
Inlet separator
gas burner
heat exchanger,
surge tank
Flash separator
3-phase, gas,glycol,condensate
From Sivalls, “Glycol Dehydration Design,” LRGCC, 2001
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
22
Field Glycol Unit
stripping still
contactor
reboiler
From Wind River Environmental Group
John Jechura – jjechura@mines.edu
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Common Operational Problems
• Contactor foaming
 Contaminates: hydrocarbons, salts, particulates, inhibitors, O2
• Poor dehydration other than from foaming
 Gas rate too low - 80% flow reduction = 20 % tray eff
 Glycol rate low - 75% flow reduction = 33% tray eff
 Glycol inlet temperature too high
• Flash drum / Still foaming
 Heavy hydrocarbons
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
24
Topics
• Introduction
• Water Content of Hydrocarbons
• Gas Dehydration Processes
 Absorption processes
 Adsorption processes
 Non regenerable desiccant processes
 Membrane processes
 Other processes
 Comparison of dehydration processes
• Safety and Environmental Considerations
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
25
Absorption vs Adsorption
Absorption
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Adsorption
26
Physical absorption
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Physical absorption
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Adsorption fundamentals
• Two types of adsorption
 Chemisorption
• Chemical interaction between adsorbate and adsorbent
• May not be completely reversible
 Physical adsorption
• Only physical interaction between adsorbate and adsorbent
• Completely reversible
-ΔHChem >> -ΔHPhys
John Jechura – jjechura@mines.edu
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Adsorption fundamentals
Consider only physical adsorption
• Factors affecting selectivity
 Size – adsorbent pore diameter major factor
 Volatility – less volatile displaces more volatile (e.g., C3 displaces C2)
 Polarity
• For desiccants, more polar displaces less polar (e.g., CO2 displaces C2, MeOH displaces CO2,
water displaces MeOH)
John Jechura – jjechura@mines.edu
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Adsorption Isotherms
Lb Water Adsorbed / 100 lb Activated Adsorbent
25
20
15
32 °F
100 °F
77 °F
400 °F
150 °F
600 °F
500 °F
10
5
0
1.0 E-8
1.0 E-7
1.0 E-6
1.0 E-5
1.0 E-4
1.0 E-3
1.0 E-2
1.0 E-1
1.0 E0
1.0 E+1 1.0 E+2 1.0 E+3 1.0 E+3
Partial Pressure of Water, psia
From UOP
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Example Solid Desiccant Dehydrator Twin Tower System
Fig. 20-76, GPSA Engineering Data Book, 13th ed.
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Zeolite structures
Zeolite A
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Zeolite X
33
Typical Vessel Loading
floating screen
24,000 lb
MS 4A
4x8 mesh
(1/8 inch)
bed diameter,
15.6 ft
bed height,
6.5 ft
fixed screen
6 in of 1/2 in
diameter
ceramic balls
3 in of 1/2 in
diameter
ceramic balls
3 in of 1/4 in
diameter
ceramic balls
Possible configuration for drying 100 MMscfd to a dew point
of -150ºF, adsorption time ~12 hours
John Jechura – jjechura@mines.edu
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Concentration Profile
Equilibrium Zone (Saturated)
Mass Transfer Zone (Partially
saturated)
Active Zone (Unsaturated)
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Concentration Profile
Equilibrium
Zone
Active
Zone
Vapor Phase Concentration
yIn
Mass
Transfer
Zone
yOut
0
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Updated: March 29, 2016
Bed Length
L
36
Concentration Profile
Dry
Break-Through
Saturated
Vapor Concentration
yIn
yOut
0
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Updated: March 29, 2016
Time
37
Regenerating Bed Temperature History
Heat On
600
Inlet Temperature
300
250
400
200
300
150
100
Outlet Temperature
200
Temperature, ºC
Temperature, ºF
500
50
100
Desorption
Bed Cooling
Bed Heating
0
0
1
2
3
4
5
6
7
8
Time, Hours
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38
Regenerating Bed Temperature History
Heat On
600
300
Inlet
Temperature
250
400
200
Outlet
Temperature
300
150
100
200
Temperature, ºC
Temperature, ºF
500
50
100
Desorption
Bed Heating
Bed Cooling
0
0
1
2
3
4
5
6
7
8
Time, Hours
John Jechura – jjechura@mines.edu
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39
Common Adsorbents for Drying
In order of increasing cost
• Silica gel (SiO2)
 Min exit water content 10 to 20 ppmv (~-60oF)
 Inert and used for inlet concentrations of > 1 mol%
• Activated Alumina (Al2O3)
 Min exit water content 5 to 10 ppmv (~-100oF)
 High mechanical strength but more reactive
• Molecular Sieve (4A and 3A)
 Min exit water content < 0.1 ppmv (~-150oF)
 Highest surface area
 Composite of sieve and clay binder
John Jechura – jjechura@mines.edu
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Design steps
• Determine size of vessels for adsorption
 Determine the bed diameter based on allowable pressure drop
• Too small – pressure drop will be too high & can damage the sieve
• Too large – too large of regeneration gas rate to prevent channeling
• Typically use (-∆P/L) < 0.33 psi/ft with a total pressure drop of 5 – 8 psi max
 Choose an adsorption period & calculate the mass of desiccant
• Sets the bed height – contributions from saturation zone & mass transfer zone heights
• Too long – more desiccant & larger vessels needed than necessary
• Too short a time – shorter desiccant life
• 8 to 12 hour periods with 2 or 3 beds are common
• Regeneration
 Calculate heat required to desorb water while also heating the desiccant & vessel
 Total amount of regeneration gas flow calculated based on heating phase about 50-60% of total
regeneration time
 Regeneration gas flowrate should give a pressure drop gradient of at least 0.01 psi/ft
John Jechura – jjechura@mines.edu
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Design equations
• Determine bed diameter
 Modified Ergun equation for pressure drop
∆P
= BµV + CρV 2
L
• Viscosity [cP] & density [lb/ft³] determined at inlet
conditions
• Solve quadratic equation for maximum superficial velocity (V [ft/min])
• Pressure drop gradient in units of psi/ft
 Minimum diameter
Dmin =

4 m
π ρVmax
 Adjust diameter upwards to nearest ½ foot increment
John Jechura – jjechura@mines.edu
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42
Design equations
• Determine bed length
 Amount of desiccant in saturation zone
=
Ssat
mwater
0.13 C SS CT
⇒=
Lsat
4 Ssat
π D2ρbulk
Assumes 0.13 lb water per 100 lb dessicant
 Amount of desiccant in the mass transfer
zone (MTZ) (GPSA Data Book method)
 V [ ft/min] 
LMTZ [ ft ] = 
CZ
35


where CZ is 1.70 ft for 1/8 inch sieve &
0.85 for 1/16 inch sieve
=
C SS 0.636 + 0.0826 ln ( %sat )
 Trent method for MTZ
LMTZ [ ft=
] 2.5 + 0.025 V [ft/min]
CT =
1.20 − 0.0026 ( °F )
John Jechura – jjechura@mines.edu
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43
Design equations
• Determine bed length (cont.)
 Total bed height is Lsat+LMTZ but should not be less than the bed diameter
 Total bed pressure drop should be 5 – 8 psi max
• If too large increase the bed diameter
• Determine vessel height & weight
 Total bed height plus other allowances – minimum 3 ft for inlet distributor on top and
bed support & hold down balls underneath
• Regeneration Calculations
 Heat loads
• Heat to desorb water – water heat of vaporization @ regeneration temperature
• Heat to increase sieve to regeneration temperature
• Heat to increase vessel to regeneration temperature
• Heat losses – typically estimated as 10%
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Design equations
• Regeneration Calculations (cont.)
 Calculation of vessel weight for heating calculations
=
t [in]
12 D Pdesign
37600 − 1.2 Pdesign
+ 0.0625
msteel [lb] =+
155 ( t 0.125 ) ( Lvessel + 0.75 D ) D
where the 0.75D term accounts for the weight of the vessel heads
 Regeneration temperature usually 50oF less than the hot gas inlet temperature
 Total regeneration load 2.5 times the minimum load
• Assumes only 40% of the heat in the regen gas actually is transferred to water, sieve, etc.
• The remainder exits as hot gas.
 Regen gas flowrate. Check that pressure drop gradient at least 0.01 psi/ft
 Regen Gas
=
m
QTotal Regen
C P (Thot − Tbed )
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
=
⇒ Vrg
 rg 4
m
ρrg πD2
45
Common Operational Problems
• Loss of bed capacity
 Aging, rapid initial loss then gradual loss over years
 Coking by partial oxidation of heavy hydrocarbons
 Coking by conversion of H2S to elemental sulfur
 Poor regeneration
• Increased pressure drop
 Attrition
 Caking at top of bed
• Fines
 Attrition
 Failed bed support
• COS formation
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Other processes
• Consumable salts (CaCl2)
• Refrigeration with MEOH addition, more complex
• Membranes, ideal for remote sites when low pressure permeate gas can be used
effectively
• If drying high pressure gas:
 Vortex tube – one application known
• Simple but poor turndown ratio and efficiency
 Twister Supersonic Separator one known offshore application
• Simple, poor turndown ratio but better efficiency
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47
Twister Operating Principle
• Acceleration to Mach >1 cools gas (typically 60 – 80oC) ΔP = 30%
• Cooling causes condensation (water and heavier hydrocarbons)
• Swirl centrifuges liquid droplets to the tube wall
• Drainage section removes liquid film from the wall + ~20% gas
• Diffuser section recompresses the gas
http://twisterbv.com/PDF/resources/Twister_-_How_Does_It_Work.pdf
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Updated: March 29, 2016
48
Comparison of Dehydration Processes
• For < 1 ppmv H2O need mole sieve.
• For higher concentrations:
 Glycol (usually TEG) widely used
• Minimal manpower requirements
• High turndown
 Regenerative desiccants (silica gel, alumina) more costly
 Membranes, and Twister(?) where pressure drop acceptable
 Nonregenerative desiccants (CaCl2) for remote, low water content gas
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
49
Topics
• Introduction
• Water Content of Hydrocarbons
• Gas Dehydration Processes
 Absorption processes
 Adsorption processes
 Non regenerable desiccant processes
 Membrane processes
 Other processes
 Comparison of dehydration processes
• Safety and Environmental Considerations
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
50