SKN 3113 2010/2011
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HYDROCARBON GAS DEHYDRATION
• Why dehydration (chap. 3) ? (Video Clip : Fire from ice)
• Gas hydrate prevention methods
– Hydration prevention (Chap 3)
– Dehydration
• Refrigeration (low temperature separation, e.g.
Joule-Thomson Expansion) – Chap 4
• Absorption (Liquid Desiccant Dehydration)
• Adsorption (Solid Desiccant Dehydration)
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ABSORPTION DEHYDRATION
• Water removal process from gas mixture stream by
contacting a gas stream with a liquid desiccant / absorbing
agent (e.g. glycol solution) which has a greater affinity for
water than gas
• Contacting takes place at an elevated pressure (usually that
of gas pressure to be used/sold ~ 60-70 bar))
• After contacting gas, water rich liquid desiccant /
absorbing agent is regenerated by heating (usually at
atmospheric temp.)
• liquid desiccant / absorbing agent is then cooled and re-
circulated back to the contactor
• Glycol dehydration is the most commonly applied method as it can lower the
dew point temperature by about 56 oC – adequate to meet most requirement
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Gas Processing and Separation 3
BASIC FLOW DIAGRAM OF ABSORPTION
DEHYDRATION
Inlet Scrubber
Wet Gas Inlet
Condensate Outlet
Contactor Tower
Glycol Cooler
Dry Gas
Outlet
High Pressure Filter
Reboiler
Surge Tank
Three Phase Gas, Glycol &
Condensate Separator
Excess Separated
Gas Outlet
Condensate Outlet
Wet Glycol from Absorber (High Pressure)
Wet Glycol to Reboiler (Low Pressure)
Dry Glycol from Reboiler (Low Pressure)
Dry Glycol to Absorber (High Pressure)
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LIQUID DESICCANT / ABSORBENT(refer to summary of adv’s & disadv’s of absorption liquids)
• High affinity for water (very much fall in love with water !!!)
• Low cost
• Non corrosiveness
• Stability (non-reactive) towards gas components
• Stability during regeneration (no/little thermal decomposition)
• Ease of regeneration ( easily separated from water)
• Low or moderate viscosity (less prone to foaming)
• Low vapour pressure at contact temperature (exist in liquid
form)
• Low solubility for natural gases and hydrocarbon liquids (hate
these H/C components !!!!)
• Low Foaming or emulsifying tendencies (no mushy solution –
interrupt flow dynamics in absorber / gas line)
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Gas Processing and Separation 7
COMPARISON OF LIQUID DESICCANTS
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GLYCOL DEHYDRATION PERFORMANCE
• Temperature of inlet gas
– …..amount of water present in inlet gas
• Temperature of inlet glycol
– Inlet glycol should be a few degree higher than inlet gas
• Lower temp than gas will cause foaming due to over
cooling & H/C condensation
• Higher temp might increase glycol loss
• Inlet gas pressure
– …….efficiency of absorption
– High contact / tower pressure
– Lower pressure drop between inlet and outlet
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• Lean glycol concentration
– …….outlet water concentration in processed gas
– …….higher glycol purity gives greater dew point
depression
• Glycol concentration (EG, DEG or TEG ???)
– ………….efficiency of absorption
– see comparison of liquid desiccants
• Number of trays in the absorber
– …….efficiency of absorption
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REGENERATION OF GLYCOL
• Reboiler temperature
– Glycol loss & thermal decomposition
• EG, DEG less than 150-160 oC
– Lean glycol concentration
• ~ 204 oC at atmospheric pressure for 98.4-98.7 w/w % TEG
• Reboiler pressure
– ….amount of water in lean glycol (influence partial pressure of
water vapor), e.g. ~ 204 oC at atmospheric pressure for 98.4-98.7
w/w % TEG
Purity of lean glycol can be increased by
- lowering reboiler pressure (lower reboiler temp.)
- using stripping gas (dry, low pressure gas, insoluble with water &
thermally stable such as methane)
- azeotropic agent such as liquid iso-octane
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OPERATING PROBLEMS
• Glycol loss
– Foaming
• H/C (aromatic) contamination
• Degradation (contribute to foaming & hence glycol loss)
– thermal decomposition
– accelerated by present sulphur
• Corrosion
– glycol oxidation
– accelerated by present sulphur
– salt contamination
• Erosion
– Sludge
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COMBATING HYDRATE FORMATIONGLYCOL INHIBITION vs GLYCOL DEHYDRATION
• Glycol inhibition
– Injecting glycol solution in gas line to depress hydrate point
– Recover lean glycol solution through separation of water from water-rich glycol in regeneration system
– Lean glycol solution concentration ~ 90-95 w/w %
– Concentration of glycol to be injected depends on oC depression of hydrate point
• Glycol dehydration
– Removal of water from gas stream by contacting water-rich gas stream with absorbent (glycol solution) in an absorber
– High pressure
– Recover lean glycol solution through separation of water from water-rich glycol in regeneration system
– Lean glycol solution concentration > 98.5 w/w %
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INJECTION ABSORPTION
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ADSORPTION DEHYDRATION
• Water removal process from gas mixture stream by contacting a gas stream with a solid desiccant / adsorbing agent
• Two main sections
– Adsorption
– Regeneration
• Heating cycle
• Cooling cycle
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Gas Processing and Separation 15
ADSORPTION SYSTEM
GPP 5/6 - Dehydration
Gas Processing and Separation 16
ADSORPTION
TOWER
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ADSORPTION SECTION
• Wet gas contact dry solid desiccant (in horizontal or
vertical (most common) bed) and consequently water
vapor held on its surface
• gas should enter from top of the column and flows
downward at above its hydrate point
• Temperatures < 50 oC & high pressure (e.g. 60-70 bar), -
no effective pressure limitation
• Batch / semi-batch operation for 8 hour cycle (most
common) – can be 6, 12 & 24 hrs
• Low dew point dehydrated gas (e.g. – 34 oC)
• Desiccant life ~ 1-3 yrs (in absence of poisoning)
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REGENERATION SECTION
(Heating Cycle)
• Water is removed from water rich solid desiccant by
heating at bed temperatures between 175-230 oC
• Heat requirement = heat of desorption of water (i.e. latent
heat of water) + sensible heat to bring water vapor
• Heating medium can be carrier hot gas (~ 200 – 260oC)
such portion of main gas stream(5-15%), or superheated
steam, usually counter-current to main flow in vertical
tower
• Water driven off at 116-120 oC
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REGENERATION SECTION
(Cooling Cycle)
• Once desired bed temperature is achieved, the bed will be
cooled (to ~ 50-55 oC) by flowing unheated/cool gas
through the bed
• Cooling less than 50-55 oC will result in water pre-
saturation of the bed (from wet gas stream) & hence
reduction next cycle adsorption efficiency
• Water vapor leaving the bed will be condensed and
separated from the regenerated/ cool gas
• For normal 8 hour regeneration cycle
– 6 hours for heating
– 2 hours for cooling
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ADSORPTION PROCESS
• Equilibrium zone – desiccant
is saturated with water (last
zone to form)
• MTZ zone – concentration
gradient exists and when the
leading MTZ reaches the end
of the bed, breakthrough
occurs
• Active zone – desiccant has
its full capacity of water
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Mass Transfer Zone (MTZ)
Single zone movement through the bed with time
• At time (1) – it is forming
• At time (3) – it is fully formed
• At time (6) – the front of the zone at the bed outlet (breakthrough)
• Between time (3) & (6) – velocity is almost constant
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ADSORPTION DEHYDRATION
& HYDROCARBON ADSORPTION
• All adsorbable components are
adsorbed at different rates with its
own adsorption zones appearing after
some interval period – representing
length of adsorption tower of each
component
• Behind the zone – all particular
component entering adsorbed on the
bed
• Ahead of the zone – concentration of
that particular component is zero
• These zones form and move down
through the desiccant bed and would
be displaced almost entirely by the
components in the zone following it
down the bed if the cycle is continued
• Water would be the last zone formed
Wet gas
Dry gas
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DESIGN OF SOLID DESICCANT ADSORBER
• Cycle time
– 8 hr cycle, 12 hr ????
• Allowable gas flow rate
– down flow vs upflow
• Desiccant capacity
– effective length of bed
• Required outlet water dew point
• Total amount of water to be removed
• Dynamic adsorption performance of desiccant tower
• Regeneration requirements
• Pressure drop limitation
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SOLID DESICCANT
• Large surface area for the high capacity (i.e. 500-800
m2/gram)
• Possess "activity" for the components to be removed
• Mass transfer rate is high
• Easily and economically regenerated (reversability of
adsorption and ease of regeneration)
• Good activity retention with time
• Small resistance to gas flow
• High mechanical strength
• Fairly cheap, non-corrosive, non-toxic, chemically inert
and possess high bulk density
• No appreciable change in volume during adsorption and
desorption
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SOLID DESICCANT PROPERTIES
• Bauxite (Al2O3 )
– ~ outlet dew point - 73oC most
– 4-6 kg water per 100 kg of desiccant expensive
• Alumina
– 4-7 water per 100 kg of desiccant
• Gels (Silica or Alumina)
– ~ outlet dew point - 60oC
– Surface area ~ 750-830 m2/gram
– 7-9 kg water per 100 kg of desiccant
• molecular sieves (Zeolite)
– ~ outlet dew point - 90oC
– Surface area ~ 650-800 m2/gram
– 9-12 kg water per 100 kg of desiccant
• Charcoal (not used for dehydration
– possess negligible water capacity) leastexpensive
Gas Processing and Separation 26
Nominal diameter of common molecules
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DESICCANT PROPERTIES AFFECTING
ADSORPTION PERFORMANCE
The pore opening at the surface of the desiccant must be
large enough to admit the molecules being adsorbed to
the interior of the particle where most of the surface area
exist
The affinity of the molecules being adsorbed must be of
the same polarity ( i.e. polar-to-polar & non-polar to non-
polar) – electric charges on the inner surfaces of the
desiccant (e.g. molecular sieves) are attracted to similar
charges on polar molecules (e.g. water)
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SOLID DESICCANT DEGRADATION
Loss of capacity occurs when the external surface of the
desiccant becomes coated with materials such as amines,
glycol, heavy hydrocarbons, elemental sulphur & liquid
water which block access to the large interior surface
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GPP 5 & 6 – ADSORPTION DEHYDRAION
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FEED GAS
P ~ 70 bara
T=30oC
TREATED
GAS
~ 0.076 mol
% H2O
Gas Processing and Separation 32
SKN 3113 2010/2011
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Gas Processing and Separation 33
