Hydrate formation represents a significant risk to process safety as it can result in the plugging of both pipes and instruments. Hydrates typically form in process where light hydrocarbons, water vapor and low temperatures or high pressures are present. This article describes the conditions under which hydrates form, how formation may be prevented and what can be done once hydrates have formed.
Generally speaking hydrate crystals are molecules of a light hydrocarbon surrounded by a crystal structure of water molecules. The classic example is a molecule of methane surrounded by water molecules. There are several crystal structures that may form, depending on the composition of the gas.
- Structure I - formed with smaller molecules, CH4, C2H6, CO2, H2S. The water structures may be either 12 sided dodecahedrons or 14 sided tetrakaidecahedrons.
- Structure II - formed with larger molecules, C3H8, C4H10 and N2. The water structures will be a combination of smaller 12 sided dodecahedrons and larger 16 sided hexakaidecahedrons.
- Structure H - formed with certain isoparaffins and cycloalkanes. They also require the presence of smaller molecules, such as methane, to form smaller hydrates which can be built up into the larger hydrate structure. The water structures will be a combination of 12 sided dodecahedrons, irregular dodecahedrons and 20 sided irregular icosahedrons.
The appearance for these hydrates in bulk is somewhat similar to that of normal ice.
Hydrate Formation Conditions
There are several factors that strongly influence hydrate formation, and several that have a more minor effect. The factors that strongly effect hydrate formation are:
- Dew point - the gas must be at or below the dew point for hydrates to form.
- Low Temperature
- High Pressure
- Gas Composition
Factors with a more minor effect on hydrate formation are:
- Nucleation sites
Hydrate formation is strongly correlated to fluid composition, so care must be taken when generalizing or extrapolating data related to hydrate formation.
The chart above shows the hydrate formation conditions for pure methane, and a 10% ethane-methane mix.
Consequences of Hydrate Formation
There are several potential consequences of hydrate formation of varying degrees of severity these may include:
- Reduction of flow in pipe lines
- Blocking of pipe lines
- Fouling of equipment
- Blocking of instruments
- Trapping pockets of fluid or pressure
These consequences range in severity from nuisance efficiency losses, such as the restriction flow in a line, or the fouling of a heat exchanger all the way to critical hazards, such as blocking safety critical instrumentation or valves.
Hydrate Prevention and Control
The prevention of hydrate formation is preferable to remediation to ensure safety and efficiency of the plant is maintained in addition to increased difficulty and cost of remediation relative to prevention. Some common hydrate prevention techniques are described as follows.
Where suitable, a temperature control system can be implemented to keep the temperature of the gas above the dew point as hydrates will not form below this temperature. A specific dew point monitoring or moisture analyzing device can be used to aid the temperature control.
Water Bath Heater
A heater may be used to prevent gas from reaching it's dew point, this is particularly useful when the expected temperature drop is known in advance. For example during pressure let down through a control valve, a water bath may be use to pre-heat the gas before the valve so that the final temperature leaving the valve is above the dew point.
Reduction of the quantity of water vapor in a gas will lower the dew point and therefore lower the likelihood of hydrate formation. Several dehydration technologies are available including:
- Molecular sieves - typically a silicate compound with very small pores which can trap water molecules selectively.
- Glycol dehydration - typically triethylene glycol (TEG) although diethylene glycol (DEG), ethylene glycol (MEG) and tetraethylene glycol (TREG) may also be used.
Depression of the hydrate formation temperature can be achieved through the injection of thermodynamic inhibitors such as methanol or ethylene glycol (MEG). These inhibitors are usually required to be injected at a high rates, typically 40-60 wt% of the water content.
Kinetic Rate Inhibitors and Anti-agglomerates
Kinetic rate inhibitors and anti-agglomerates are usually surface-active compounds, polymers and copolymers with surfactant properties. Kinetic rate inhibitors greatly reduce the rate of formation of hydrates. Anti-agglomerates prevent the hydrates for combining together and attaching to fixed surfaces, allowing them to remain transportable through a pipeline and removed in a convenient location.
These are several steps which may be employed to remove hydrates once formed. These can be implemented individually or in combination. Care must be taken when decomposing hydrates as there are several risks associated with their removal:
- Hydrate dissociation can lead to the rapid release of water vapor and gas, which can dramatically increase the pressure in a closed system.
- Multiple hydrate plugs may trap pressure and flammable fluid between them.
Heating and Pressure Reduction
Dissociation of hydrates can be promoted through the application heat or reductions in pressure.
Methanol or glycol injection can be used to break down the hydrates. The conditions under which this is a appropriate strategy depends on the positioning of the hydrates as the injected fluid must have direct contact with the hydrate formation. For example, it is unlikely to be economical to use this strategy to remove hydrates from the circumference of a long horizontal pipeline due to the requirement to fill the pipe completely.