Natural Gas Dehydration

Pipeline Specification and the Hydrate Risk

Gas produced from a reservoir is saturated with water at reservoir temperature and pressure. As that gas cools and is pressurised in the pipeline, the water condenses, freezes against the pipe wall as ice, or combines with light hydrocarbons to form solid hydrates that block valves, instruments, and pig launchers. A sales gas pipeline therefore enforces a contractual water dew point, typically between minus 10 °C and minus 40 °C at line pressure, and a cryogenic gas plant tightens that limit to below minus 70 °C to protect the cold box.

Triethylene glycol contactors remove bulk water economically but cannot reach a low enough outlet dew point for cryogenic service. SorbiTech delivers adsorption on a molecular sieve bed, the only proven technology that meets the full specification reliably at high pressure and over a wide temperature range.

How Adsorption Dehydration Works

Wet gas passes through a packed bed of molecular sieve, which holds water on its internal surface down to single digit ppm. The mass transfer zone moves slowly down the bed during the adsorption stage. When it reaches a predefined penetration depth the bed is taken offline, heated to 250 to 320 °C with a dry purge to drive off the captured water, then cooled with the same purge and returned to service.

Two adsorbers running 180 degrees out of phase give a continuous flow of dry gas; a third vessel is added for sour service or for half cycles longer than 24 hours.

Sizing the Cycle

The bed volume and the cycle time are set by the inlet conditions and the contracted outlet:

• Inlet water content at saturation, pressure, and temperature
• Required outlet dew point: minus 10 °C (sales gas) to below minus 70 °C (cryogenic feed)
• Adsorption half cycle: typically 8 to 24 hours
• Mass transfer zone length, which sets the over design margin on bed volume
• Regeneration purge flow and heater duty
• Operating pressure, which sets the vessel wall thickness and the heat duty

A guard layer of activated alumina or silica gel is added at the inlet when the inlet water load is unusually high or when liquid carryover from the upstream separator cannot be fully ruled out.

Choosing the Sieve Grade

The pore size of the sieve is matched to the gas composition. The SorbiTech adsorbent line covers the full range:

• Molecular Sieve 3A is selected for sales gas and for any stream containing ethane, propylene, or heavier hydrocarbons. The 3 Ångström pore excludes those species and prevents coadsorption losses and coke laydown.
• Molecular Sieve 4A is selected for pure methane and for cryogenic feeds where maximum water capacity is the priority.
• Molecular Sieve 13X is selected when the duty also requires carbon dioxide and mercaptan removal in the same bed, and is the default choice for cryogenic air separation pre purification.

Common Failure Modes and Mitigations

Three failure modes recur in TSA dehydration. First, liquid carryover from the upstream separator wets the sieve and causes hydrothermal damage during the first regeneration cycle. The mitigation is a correctly sized inlet coalescer with a continuous liquid level alarm.

Second, an undersized regeneration heater means the bed never reaches the temperature required to fully drive off the captured water. Outlet dew point drifts upward until the campaign is cut short. The SorbiTech mitigation is a heater duty calculation that allows for the highest expected water load plus a margin of 10 to 15 percent.

Third, channelling at the vessel wall caused by poor flow distribution leaves part of the bed unused and accelerates breakthrough. The mitigation is a SorbiTech vessel internal design with a tested distributor and a height to diameter ratio between 1.5 and 4.

Delivery

SorbiTech sizes and delivers the duty as a complete twin tower TSA dehydration unit, with the SorbiTech media and the SorbiTech equipment under one performance guarantee. SorbiTech delivers turnkey for projects that include civil, electrical, and commissioning scope. Across the SorbiTech Group the duty is served by the twin tower TSA package charged with 3A or 4A media, with sector coverage under oil & gas and petrochemicals and refining.