Resembling dirty ice, hydrates consist of a water lattice in which light hydrocarbon molecules are embedded. They are a loosely-linked crystalline chemical compound of hydrocarbon and water called cathrates, a term denoting compounds that may exist in stable form but do not result from true chemical combination of all the molecules involved. Hydrates normally form when a gas stream is cooled below its hydrate formation temperature. At high pressure these solids may form at temperatures well
above 32°F. Hydrate formation is almost always undesirable because the crystals may cause plugging of flow lines, chokes, valves, and instrumentation; reduce line capacities; or cause physical damage. This is especially true in chokes and control valves where there are large pressure drops and small orifices. The pressure drops cause the temperature to decrease, and the small orifices are susceptible to plugging if hydrates form. Hydrate formation leading to flow restrictions is referred to as “freezing.”
The two major conditions that promote hydrate formation are (1) the gas being at the appropriate temperature and pressure, and (2) the gas being at or below its water dew point with “free water” present. For any particular composition of gas at a given pressure there is a temperature below which
hydrates will form and above which hydrates will not form. As the pressure increases, the hydrate formation temperature also increases. If there is no free water, that is, liquid water, hydrates cannot form. Secondary conditions such as high gas velocities, agitation of any type, and the formation
of a nucleation site may also help form hydrates. These secondary conditions are almost always present in the process piping stream.
Methods of preventing hydrate formation include adding heat to assure that the temperature is always above the hydrate formation temperature, lowering the hydrate formation temperature with chemical inhibition, or dehydrating the gas so that water vapor will not condense into free water. It is also feasible to design the process so that if hydrates form they can be melted before they plug equipment.
Before choosing a method of hydrate prevention or dehydration, the operating system should be optimized so as to minimize the necessary treating. Some general factors to consider include the following: (1) reduce pressure drops by minimizing line lengths and restrictions, (2) take required pressure drops at the warmest conditions possible, and (3) check the economics of insulating pipe in cold areas.
This chapter discusses the procedures used to calculate the temperature at which hydrates will form for a given pressure (or the pressure at which hydrates will form for a given temperature), the amount of dehydration required to assure that water vapor does not condense from a natural gas stream, and the amount of chemical inhibitor that must be added to lower the hydrate formation temperature. It also discusses the temperature drop that occurs as gas is expanded across a choke. This latter calculation is
vital to the calculation of whether hydrates will form in a given stream.
The next chapter discusses the use of LTX units to melt the hydrates as they form, and the use of indirect fired heaters to keep the gas temperature above the hydrate formation temperature.