Gas Dehydration
Chapter 11
Based on presentation by Prof. Art Kidnay
John Jechura – jjechura@mines.edu
Updated: March 29, 2016
Plant Block Schematic
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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
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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:
yi
x i K i where K i
i ,L
i ,V
P vap
exp
P
i ,V
P
i i
Pi
sat
vi ,L
RT
dP
For a gas in contact with pure water:
yH2O
vap
PH2O
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
Wet Basis: XH2O
Dry Basis: XH2O
NH2O / MH2O
NHC NH2O
NH2O / MH2O
NHC
yH2O / MH2O
yH2O / MH2O
1 yH2O
At very low water contents these values
are essentially the same
Fig. 20-4, GPSA Engineering Data Book, 13th ed.
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Water content of natural gas – typical pipeline specs
GPSA Engineering Data Book, 13th ed.
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Water content of natural gas
GPSA Engineering Data Book, 13th ed.
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Applicability of dehydration processes
1000
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
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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:
yH2O
xH2O
vap
PH2O
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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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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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
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Typical Glycol Dehydration Unit
System
2 – 5 gal TEG per lb water removed
Absorber / Contactor
Regenerator Temperatures
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
Absorber Temperatures
Packed equivalent to 3 – 4 trays
375 – 400oF
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yH2OP
vap
xH2OPH2O
20
Solubility of hydrocarbons in glycol solutions
GPSA Engineering Data Book, 13th ed.
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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
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Common Operational Problems
Contactor foaming
Contaminates: hydrocarbons, salts, particulates, inhibitors, O 2
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
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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
-
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Chem
>> -
Phys
29
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., CO 2 displaces C2, MeOH displaces CO 2,
water displaces MeOH)
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Adsorption Isotherms
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
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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
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Concentration Profile
Equilibrium Zone (Saturated)
Mass Transfer Zone (Partially
saturated)
Active Zone (Unsaturated)
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Concentration Profile
Mass
Transfer
Zone
Equilibrium
Zone
yIn
yOut
0
Active
Zone
L
Bed Length
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Regenerating Bed Temperature History
Heat On
600
300
500
Inlet
Temperature
250
400
200
Outlet
Temperature
300
150
100
200
50
100
Desorption
Bed Heating
Bed Cooling
0
0
1
2
3
4
5
6
7
8
Time, Hours
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Common Adsorbents for Drying
In order of increasing cost
Silica gel (SiO2)
Min exit water content 10 to 20 ppmv (~-60 oF)
Inert and used for inlet concentrations of > 1 mol%
Activated Alumina (Al2O3)
Min exit water content 5 to 10 ppmv (~-100 oF)
High mechanical strength but more reactive
Molecular Sieve (4A and 3A)
Min exit water content < 0.1 ppmv (~-150 oF)
Highest surface area
Composite of sieve and clay binder
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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
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Design equations
Determine bed diameter
Modified Ergun equation for pressure drop
P
L
B V C V2
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
Vmax
Adjust diameter upwards to nearest ½ foot increment
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Design equations
Determine bed length
Amount of desiccant in saturation zone
Ssat
mwater
0.13 C SS CT
Lsat
4 Ssat
D2 bulk
Assumes 0.13 lb water per 100 lb dessicant
Amount of desiccant in the mass transfer
zone (MTZ) (GPSA Data Book method)
LMTZ ft
C SS
0.636 0.0826 ln %sat
V ft/min
CZ
35
where CZ is 1.70 ft for 1/8 inch sieve &
0.85 for 1/16 inch sieve
Trent method for MTZ
LMTZ ft
2.5 0.025 V ft/min
CT
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1.20 0.0026 F
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
msteel lb
0.0625
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 50 oF 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
mRegen Gas
QTotal Regen
C P Thot Tbed
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Vrg
mrg 4
D2
rg
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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Twister Operating Principle
Acceleration to Mach >1 cools gas (typically 60 – 80oC) P = 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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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
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