4 Hydrates & Hydrate Prevention

EP 20267_b_A_ppt_02 - HYDRATES & HYDRATE PREVENTION

HYDRATES AND HYDRATE PREVENTIONPART 2 : HYDRATE FORMATION CONDITIONS

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� 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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� 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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� INTRODUCTION







COURSE OUTLINE

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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


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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HYDRATE FORMATION CONDITIONS – IFP RESERCH FACILITIES

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� INTRODUCTION







COURSE OUTLINE

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HYDRATE FORMATION CURVE – Influence of GAS COMPOSITION (Specific Gravity)

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� INTRODUCTION







COURSE OUTLINE

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EFFECT OF H2S AND CO2 ON HYDRATE FORMATION CONDITIONS – Ref. GPSA

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HYDRATE FORMATION TEMPERATURE FOR GASES CONTAINING H2S

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� INTRODUCTION







COURSE OUTLINE

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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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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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OVERVIEW OF HYDRATE PREVENTION

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� INTRODUCTION







COURSE OUTLINE

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WATER

another compound

A GAS

PRESSURECOLD

Key points hydrates formation conditions

To get some hydrates you need 4 things :

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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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HYDRATE PUG REMOVAL – SAFETY HAZARDS

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HYDRATE PUG REMOVAL – SAFETY HAZARDS

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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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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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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.

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4 Hydrates & Hydrate Prevention