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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATES AND HYDRATE PREVENTIONPART 2 : HYDRATE FORMATION CONDITIONS
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
� INTRODUCTION
� HYDRATE FORMATION CONDITIONS
� PREDICTION OF HYDRATE FORMATION
• HYDRATE FORMATION CURVES
• CASE OF SOUR GASES
• USE OF SIMULATION SOFTWARE
� HYDRATE PLUG REMOVAL
COURSE OUTLINE
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EP 20267_a_A_ppt_10 - HYDRATES & HYDRATE PREVENTION
INTRODUCTION – WHAT ARE THEY?
� Solids
� Looks like ice
� Composed of water
+
at least one other compound
IT IS NOT ICE !
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
INTRODUCTION – WHAT ARE THEY?
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
INTRODUCTION – WHAT ARE THEY?
� HYDRATES consist of hydrocarbons trapped inside lattice
of water molecules
� Increase structure STABILITY (like a ball inside a box)
� Create operating problems because, at elevated pressures,
they can exist at higher temperatures than ice (e.g; 4°C @
12 bar or 20°C @ 100 bar)
� One cubic foot of gas hydrate contains 160 cubic feet of
gas at standard temperature and pressure
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
� INTRODUCTION
� HYDRATE FORMATION CONDITIONS
� PREDICTION OF HYDRATE FORMATION
• HYDRATE FORMATION CURVES
• CASE OF SOUR GASES
• USE OF SIMULATION SOFTWARE
� HYDRATE PLUG REMOVAL
COURSE OUTLINE
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE FORMATION CONDITIONS – GENERALITIES
� 3 CONDITIONS are required for hydrate formation:
• Presence of LIQUID water (“free”, dissolved or emulsified)
• Presence of light hydrocarbons (from C1 to C4), or acid gas (CO2, H2S) or Nitrogen
• High enough P and/or Low enough T conditions (depending on the nature & composition of gas)
� May occur naturally underground, at & below seabed
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HYDRATE FORMATION CONDITIONS – GENERALITIES
EP 20267_a_A_ppt_10 - HYDRATES & HYDRATE PREVENTION
WATER
another compound
A GAS
PRESSURECOLD
Key points hydrates formation conditions
To get some hydrates you need 4 things :
Hydrogen bonds
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE FORMATION CONDITIONS – IFP RESERCH FACILITIES
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
� INTRODUCTION
� HYDRATE FORMATION CONDITIONS
� PREDICTION OF HYDRATE FORMATION
• HYDRATE FORMATION CURVES
• CASE OF SOUR GASES
• USE OF SIMULATION SOFTWARE
� HYDRATE PLUG REMOVAL
COURSE OUTLINE
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE FORMATION CURVE – Influence of GAS COMPOSITION (Specific Gravity)
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
� INTRODUCTION
� HYDRATE FORMATION CONDITIONS
� PREDICTION OF HYDRATE FORMATION
• HYDRATE FORMATION CURVES
• CASE OF SOUR GASES
• USE OF SIMULATION SOFTWARE
� HYDRATE PLUG REMOVAL
COURSE OUTLINE
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
EFFECT OF H2S AND CO2 ON HYDRATE FORMATION CONDITIONS – Ref. GPSA
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE FORMATION TEMPERATURE FOR GASES CONTAINING H2S
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
� INTRODUCTION
� HYDRATE FORMATION CONDITIONS
� PREDICTION OF HYDRATE FORMATION
• HYDRATE FORMATION CURVES
• CASE OF SOUR GASES
• USE OF SIMULATION SOFTWARE
� HYDRATE PLUG REMOVAL
COURSE OUTLINE
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
USE OF SIMULATION SOFTWARE
� These methods are based on fundamentals equations of phase equilibrium
• When hydrates form, T, P and chemical potential of H2O have the same values in gas,
liquid water and solid hydrate phases
� Computer simulation programs are superior both in accuracy and ease of
extrapolation
� Remark : PRO II does not take into account the amount of water present in the gas
• It always calculates hydrate formation assuming the presence of free water is present.
• It always assume that the gas is water saturated, i.e. it increases the gas water content if
not at water saturation
� MULTIFLASF of INFOCHEM is based on experimental data
• Accurate hydrate formation temperature of HC mixtures in presence of free water with
salts, MeOH, glycols (MEG,DEG,TEG).
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
USE OF SIMULATION SOFTWARE
- EXAMPLE OF COMPUTER SIMULATION RESULTS -
Hydrates Formation & Water Dew-Point Curves
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100
Temperatue (°C)
Pre
ssure
(bar abs)
Hydrate Curve Water Dew Point
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
OVERVIEW OF HYDRATE PREVENTION
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
� INTRODUCTION
� HYDRATE FORMATION CONDITIONS
� PREDICTION OF HYDRATE FORMATION
• HYDRATE FORMATION CURVES
• CASE OF SOUR GASES
• USE OF SIMULATION SOFTWARE
� HYDRATE PLUG REMOVAL
COURSE OUTLINE
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EP 20267_a_A_ppt_10 - HYDRATES & HYDRATE PREVENTION
WATER
another compound
A GAS
PRESSURECOLD
Key points hydrates formation conditions
To get some hydrates you need 4 things :
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE PUG REMOVAL
� Increasing the temperature to remove a hydrate blockage is not usually practical
since the blockage has to be first located and then heating equipment applied to
the blockage area. Nevertheless, this method can be applied in particular cases.
� A pipeline plugged with hydrates can be unplugged by reducing the pipeline
pressure on both sides of the plug.
� Reducing the pressure on only one side is EXTREMELY DANGEROUS!!! The solid
hydrate plug can break loose. Then the pipeline pressure drives the ice-hard
hydrate plug towards the lowered pressure side at very high velocity. When the
hydrate hits a bend or restriction, it can break the pipe and even unearth a buried
pipeline. Such accidents have resulted in loss of lives and extensive equipment
damage.
=P = 100 bar P = 70 bar
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE PUG REMOVAL – SAFETY HAZARDS
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE PUG REMOVAL – SAFETY HAZARDS
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
HYDRATE PLUG REMOVAL – EXAMPLE OF PROCEDURE
� 1st step:
• Reduce slowly by steps the pipeline pressure on both sides of the plug, one side down to a pressure near atmospheric
pressure, other side down a pressure of a few bars (< 6 bar).
• Quick gas depressurization could generate a gas cooling to negative temperature values (Joule-Thompson expansion),
which favors ice formation and extends the melting period of the hydrate plug.
• To prevent this event it is a good practice to control the gas temperature during depressurization.
� 2nd step:
• Inject methanol to the upper pressure pipeline side when the pressure value starts to decrease.
• The small flow brings the methanol towards the hydrate plug and favors by contact his dissociation.
Methanol can dissolve existing hydrates to few degrees. It is not the case with glycol: glycol can
prevent hydrates, but cannot attack or dissolve hydrates already present
Hydrate plug
4 to 5
barP
atm
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
QUIZ
True False
The presence of moisture (water at vapor state) is enough to form hydrates, when combined with gas molecules
at a low enough temperature and high enough pressure●
For hydrate formation, the presence of LIQUID water is needed
The higher the density of a gas the higher its hydrate formation temperature (at a given pressure) ●
Hydrate formation temperature increases as specific gravity of gas increases
The presence of H2S in the gas increases the hydrate formation risks ●
Presence of H2S increases hydrate formation temperature at a given pressure
Hydrate removal can be done safely by depressurization of downstream side of the flowline ●
Depressurization shall be performed gradually on BOTH sides, in order to avoid flow line rupture.
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EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION
WHAT YOU SHOULD KEEP IN MIND?
� Hydrates are composed of a water lattice, in which light hydrocarbons (C1, C2, C3, iso-C4) or impurities (H2S, CO2) are embedded
(Clathrates). The specific gravity of hydrates varies from 0.96 to 0.98
� Hydrates could:
• BLOCK flowlines, chokes, valves, instrumentation,
• FOUL and PLUG heat exchangers,
• ERODE chokes, control valves, turbo-expanders.
� The formation of hydrates in natural gas requires
1. high enough pressure and low enough temperature conditions (depending on the composition of the effluent)
2. the presence of LIQUID water
� For a given pressure, the temperature and thus the risks of hydrate formation iare higher if the density of the gas increases
� The presence of H2S increases the risks of hydrate formation
� Hydrate formation starts slowly. However, this phenomenon accelerates exponentially and ends up by blocking the
installations. Regular monitoring to detect the beginning of hydrate formation is thus more than recommended
� Eliminating a hydrate plug is hazardous and can be very difficult
� The main ideas to prevent hydrate formation are:
• Lower the Water Dew Point by removing moisture (dehydration)
• Lower the pressure (depressurization) and/or increase temperature (thermal insulation, heating…)
• Inhibition by injection of chemicals (MeOH, MEG, DEG, KHI…)
� Removing hydrates by depressurization could require days of flow interruption.