Natural gas is one of the most widely used commercial gases because it is odorless, colorless, tasteless and non-toxic. However, it needs to undergo extensive purification before it can become pipeline-quality. Water vapor is the most common undesirable impurity found in natural gas. Left untreated, it can result in the formation of ice-like hydrates, which plug flow lines and natural gas processing equipment, causing severe operational problems. Two methods employed for natural gas dehydration are expansion refrigeration, or absorption through the use of solid or liquid desiccants.
Natural gas dehydration methods, especially glycol dehydration, can be fabricated as self-contained modular skid systems. This approach is increasing in popularity throughout many industries as a flexible, cost-effective and lower-impact way of conducting process operations. This post will discuss level and flow solutions for natural gas dehydration skids and is part of an occasional Magnetrol® blog series on modular skid systems.
Glycol Dehydration Skids
The use of ethylene glycol liquid desiccants is one of the most established and reliable techniques for natural gas dehydration. Liquid desiccants include diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TETRA EG). The dehydration process is sometimes separated into two skids: one for glycol absorption and another for glycol reconditioning.
Ethylene glycol flows downward from the top of a tower and meets a rising mixture of water vapor and hydrocarbon gases. Dry gas exits from the top of the tower while the glycol/water mixture is pumped out of the bottom. The glycol and water are separated, and the glycol is recycled.
Level and Flow Applications
1. Glycol Contactor
Wet natural gas first flows through a glycol contactor to remove all liquid and solid impurities. The gas flows upward through the contactor where it is contacted and dried by glycol. The ‘pipeline-ready’ dried gas passes through a heat exchanger and into the application loop. Water-rich glycol is withdrawn from the bottom of the absorber via a level controller.
Continuous Level: Eclipse® Model 706 Guided Wave Radar Transmitter
Point Level: Tuffy® II Float Level Switch
2. Flash Tank
Skids are often provided with low pressure, three-phase flash separators to separate solution gas from the glycol and hydrocarbon condensate. A flash separator also removes up to 90% of methane emissions. The flash separator is installed on the rich glycol line between first pass of the glycol/glycol heat exchanger and the glycol filter bank.
Continuous Level: ECLIPSE Model 706 Guided Wave Radar Transmitter
3. Glycol Circulation Pump Protection
Glycol recirculation pumps operating in a reduced or no-flow condition can overheat, rupture the pump’s seal, and disrupt the glycol reconditioning circuit. A flow switch along the pump’s discharge piping will actuate an alarm and shut down the pump when liquid flow drops below the minimum flow rate.
Flow Alarm: Thermatel® Model TD1/TD2 Thermal Dispersion Switch for low-flow cutoff
4. Exhaust Gas Flow Monitoring
Exhaust gases emitted from the reboiler that are discharged directly to the atmosphere can be monitored by a mass flow transmitter. Because flow rates and gas compositions fluctuate, the mass flow transmitter can be used to obtain relative flow indication.
Continuous Gas Flow: THERMATEL Model TA2 Thermal Dispersion Mass Flow Meter