Perhaps the world’s best-known example of a phase transition occurs when water at atmospheric pressure is cooled to a temperature of 0°C (32°F). At that point, the energy in the system is no longer sufficient to keep the water molecules from bonding together. The liquid water restructures itself into a rigid solid, called ice. However, it turns out that in nature, this process is not always so clear-cut.
When pressures are suitably high, water molecules will begin to form complex solid structures at temperatures significantly above the normal freezing point. Unlike ice, these structures are characterised by regular networks of large, open cavities and are therefore inherently unstable. As cooling continues, the normally compact and stable ice structure will ultimately form, unless some outside (guest) molecule of appropriate size enters the structure and supports the cavity. In nature, the most abundant guest molecule is methane (CH4). The resulting stable, solid compound is commonly called “methane hydrate”.
In natural gas processing applications, hydrates will typically form around 15-25°C (59-77°F), depending on the operating pressure and gas composition. Hydrates blockage is a nuisance to the natural gas industry; the cost of hydrate prevention is estimated to be billions of dollars each year.
One solution to manage hydrates in natural gas processing applications is Low Temperature Extraction (LTX) technology which has been around since the early 1950s and widely applied mainly onshore in the Americas. The principle is based on a Low Temperature Separator (LTS) with heating coils in the liquid section to melt hydrates. This allows operation below the hydrate formation temperature without the use of chemicals.
Today conventional LTX technology is rarely used and has been mainly superseded by glycol-based LTS technology. The principal limitations of conventional LTX technology are the relatively bulky vessel design, poor separating performance, relatively low liquid (NGL) yield and high pressure drop. Whereas gas/vapour separators today are fitted with internals to enhance separation efficiency and reduce vessel size, such internals would freeze-up with hydrates inside an LTX separator.
Twister BV has now adapted conventional LTX technology in order to improve the design to enable efficient chemical-free gas processing in combination with the Twister® Supersonic Separator.
Twister BV has developed an enhanced version of the conventional LTX technology, the Hydrate Separator. Key features are the tangential inflow nozzles swirling the feed stream. The resulting centrifugal forces separate the heavier liquids and hydrates from the vapour, resulting in a high separating efficiency without internals (which could freeze-up with hydrates). Liquids and hydrates are collected in the bottom section, where hydrates are melted by heating coils. An additional improvement is segregation of the top separation section from the bottom liquid heating section by means of a deflector, minimising re-evaporation and thereby optimising dewpointing performance and NGL extraction. CFD modelling, field testing and commercial operation have confirmed a very high separating efficiency (>99%) and stable operation without the use of chemicals. The Hydrate Separator can be designed as either a 2 or a 3-phase separator to suit the specific requirements of each application.
The figure below shows a typical Process Flow Diagram of a Hydrate Separator integrated within a Twister dewpointing system. Inlet cooling is achieved by ambient cooling and gas/gas heat exchange. Free liquids are removed in an inlet separator. Twister tubes dewpoint the gas. The Hydrate Separator receives a mixture of hydrates, liquids and gas from the Twister tubes and delivers separated gas and liquid streams free of hydrates.