At its core, gas dehydration is the removal of water from a gas stream. The two vital reasons for this are:
- Preventing Corrosion: Water in gas can lead to corrosive damage in downstream piping and equipment, compromising integrity and safety.
- Preventing Hydrate Formation: Hydrates are ice-like crystals that can form in the presence of water, leading to costly blockages in pipes, valves, and other critical equipment.
Why Glycol? The Science Behind the Dehydrator
So, why glycol? Glycol is a member of the alcohol family, and much like alcohol can dehydrate the human body, it does the same for gas. It’s an excellent absorber of water from a gas stream.
While there are three commonly used glycols – monoethylene glycol (MEG), diethylene glycol (DEG), and triethylene glycol (TEG) – triethylene glycol (TEG) is the industry’s preferred choice. The key reason? TEG has the highest boiling point, making it far easier to regenerate and reuse by releasing the absorbed water.
The Nuances of Operating Glycol Dehydration Units
“Operating glycol units is generally straightforward, as long as nothing changes,” Mark observes. The challenge arises when changes are introduced. Glycol units have a significant hold-up of glycol, meaning it can take easily 24 hours for a change to fully work its way through the system and for the glycol to reach a new equilibrium with water. This slow response time means operators often need to make a change and then wait until the next day to assess its full effect.
Regenerating Glycol and the Role of Stripping Gas
To reuse the glycol, we must separate the absorbed water. This is achieved through distillation. Since TEG boils at around 270°C and water at 100°C, this significant difference allows us to boil the glycol-water mixture, driving off the lighter water and leaving behind the heavier, purified glycol. This recovered glycol is then recycled to remove more water from the gas.
The purity of the regenerated glycol is crucial and is affected by:
- Reboiler Boiling Point: A higher boiling point in the reboiler generally leads to purer glycol (up to a limit where glycol breakdown can occur).
- Stripping Gas: This is a stream of gas introduced into the hot, dry glycol to further enhance its purity.
- System Cleanliness: Contaminants, fouling, or dirt in the reboiler can degrade the glycol’s purity.
The Critical Importance of Solution Management
Solution management is all about maintaining the quality, cleanliness, freshness, and purity of the glycol solution. Many factors can degrade it:
- Glycol Breakdown: Glycol can break down to form acidic products, pushing the solution out of its ideal operating pH range.
- Contaminants from Process Gas: Hydrocarbons carried into the glycol contactor can dissolve into the glycol and break down in the reboiler, forming unwanted byproducts.
- Mishandling: When topping up glycol, contact with oxygen must be avoided, as glycol will oxidize to glyoxalic acid, affecting the solution’s pH.
These and other issues highlight why good solution management is paramount to the efficient and effective operation of glycol dehydration systems.
Innovations: Making Glycol Processes More Efficient and Environmentally Friendly
Modern glycol dehydration units are making strides in efficiency and environmental responsibility. Historically, emissions came from two main sources:
- Glycol Flash Drum: High-pressure dissolved impurities, particularly hydrocarbons, would flash off to atmosphere or flare.
- Reboiler Still Column: Water vapours, stripping gas, and other contaminants were released to atmosphere.
Today, modern systems capture these vapours from both locations, compressing them and pushing them back into the gas stream upstream of the contactor, preventing atmospheric release.
In terms of energy efficiency, while units already utilize heat exchangers, further improvements are being made. This includes using electricity (especially from renewable sources like wind or solar) to heat the reboiler, or recovering waste heat from other systems like gas turbines or boilers, significantly boosting the unit’s energy efficiency.
ESD’s Long-Standing Expertise in Glycol Dehydration Training
ESD Simulation Training has been delivering glycol dehydration courses since the early 1990s, boasting extensive experience in this field. As with all ESD courses, simulation is a key component. The dynamic simulation models used are incredibly effective for exploring the operation and control of these complex units, providing invaluable hands-on learning.